Anodic treatment of aluminum



l I I This inventionrelates to a I protective coatings on aluminum or aluminum alloys by using the object Patented June 29, 1937 PATENT OFFICE anomc TREATMENT 0F ALUMINUM Robert ,W. :Buzzard, .Kensington, Md.

i No Drawing. Application May 22, 1934,

. Serial No. 726,896

4 Claims. (01. 204-1) (Granted under the act. of. March 3, 1883, as

amended April 30, 1 928; 370 0. 757) to be treated as the anode in ,a suitable bath. It is the object of this invention to devise a process for the purpose mentioned that will include a considerable range of concentrations of the chemically active component of the bath as well as making it possible to effect the coating over a relatively large range of temperature and voltage conditions. i

This application is a continuation in part of my copending application Serial No. 656,761.

The processes for anodic treatment of aluminum in chromic acid solutions heretofore generally used have been restricted to a very definite concentration for the bath, have necessitated a close control of a voltage cycle and also a definite, elevated temperature.

In the practice of my invention the article to be treated, after having been thoroughly cleaned, is placed in a bath containing from 4% to 25% of chromic acid. With the article serving as the anode, a constant potential from a source of direct current between volts and 50 volts is applied to the bath, which is kept at a constant temperature between C. and 60 C., for a period of about one hour. Probably the most satisfactory concentration of the ,chromic acid is 9.5%, but it may be variedwith good results between the limits above mentioned. Further, it has been found that satisfactory oxide films for definite purposes can be formed in any-concentration of chromic acid byproperly adjusting the voltage and the temperature. These discoveries are the product of a long and extensive research on the subject. They are neither obvious nor self-evident from The same results nating current.

A superior film may be formed in a chromic acid bath containing additional chromates, the chromic acid content and the percentage of chromate being varied according to the type of film desired and the particular chromate employed. The temperature and voltage used with this type of bath must be varied to suit the composition of the electrolyte but for any given type of film. the voltage is kept constant. As one specific example, there may be mentioned a bath of 5% chromic acid by weight and 5% potassium dichromateby weight'using a constant direct current voltage of 40 volts and aconstant tempera ture between 35 C. and 40 C. The film forms rapidly and a satisfactory thickness'will be decannot be produced by alterprocess for forming.

- continued for an hour.i

previous work on anodic films.

veloped in one-half hour but the corrosion resistant properties-are increased if the treatment is The film formed by a bath having the composition mentioned in the preceding paragraph is superior in some qualities to those formed in straight chromic acid baths and in general to those formed in straight chromate baths and is not thesame type of film aswould be obtained from sucha treatment in the two baths. separately since the chromate acts as a hardener of the-film. In fact, the result so obtained is not predictable from a study of the coatings produced in. that way. The use of chromic acid and a chromate together is not limited to the particular proportions mentioned as avery good film is formed in a 3% chromic acid bath with 1% of potassium dichromate. The range of concentrations for this compound bath has not been definitely delimited but it isknown that satisfactory coatings are formed at most concentrations in which the sum of the percentages of chromic acid and potassium dichromate is n t more than of the bath.

As examples of the values of voltages and temperatures that may be used successfully when the I concentration of chromic acid is in the neighborhood of 10%, the following may be mentioned: volts at 0.; 30 volts at C.; 40 volts at 35 C.; 42 volts at 40 C.; 35 volts at 40 C.; 35 volts at 35 C. Or the following cycle may be followed with the temperature held constant between 35 C. and 40 C.: Raise voltage to 40 volts in five minutes, maintain 40 volts for ten minutes, lower to 35 volts in five minutes, maintain 35 volts for ten minutes, lower to 30 volts for five minutesand maintain 30 volts for twenty-five minutes. In all cases the voltage referred to is derived from a source of direct current. When the voltage is kept constant at any value, say 40 volts, it may be raised to that value as quickly as the source of power will permit, or as slowly as the operator desires. The duration of treatment depends upon the thickness and type of film desired, the longer times, within limits, producing thicker films. Also, the higher the temperature the softer and darker the film.

It is known that the effect of lowering the temperature of the bath is to reduce the current density, whereas the effect of lowering the voltage is to raise the current density within limits. The effect of increasing the chromic acid concentration at constant voltage and temperature is to increase the current density, but also the effective temperature may be readily lowered with v increased'chromic acid concentration to counteract'this effect.

It has been found in general with the baths described herein that there is one voltage at of the film is produced by different temperature,

batch bath rather than being restrizxted to batch voltage-time cycles in different concentrations. As the concentration of thebath is increased the temperature may first be lowered and then" the time-voltage as the concentration increases. lItw is not recommended that both time and voltage be lower for the same cycle.

It is optional whether thematerial'is washed to remove therefrom the adhering solution, since if such solution is left on it will, under certain conditions, improve thecorrosion resistance... Any solutionthat remains in cracks does not attack-the: metalor :have any detrimental efiect' and when oncethe' solution is. dried on it isvery difli cult to remove. being. made of; this process bythe Navy Departmentand is covered by the Navy specification for the Anodic treatment of aluminum and its alloys. 1 I

rials both in a continuous processbathandin a treatment alone.

The invention described Iactured and used by or for the. Governmentof Commercial application is now herein may be manu- 'anodic treatment either in batches or in con-'- tinuous process, protective coatings on articles having aluminum as a constituent, comprising ,3 chromic acid and 1% potassium dichromate as a film hardener. i 3. A bath for forming with direct current, by anodic treatment in continuous process, protective coatings on articles having aluminum as a constituent, comprising irom 3% to 5% of chromic acid and 1% to5% ofpotassium dichromate as-afilm hardener. v

4. A process of forming, with direct current, anodic coatings on material having aluminum as a principal constituent, comprisingthe steps of utilizing thematerial to be treated'as an anode in a bath containing from 3% to "5% chromic It isreadily'apparent that since my bath permits .theuse of-constant voltageand constant temperature it may be used fortreating mateacid and 1% to 5% potassium dichromate as a film hardener, the temperature ofsaid bath being between 35 C'an'd 40 c and'applying to said anode and bath a potential of between 30 volts and-42' volts; the-aforesaid steps being applicable as a continuous process.

ROBERT w. BUZZAR .D[ 

